probability, the one tube with which you are most familiar. Before you started your study of electronics, you probably referred to cathode-ray tubes as picture tubes. The cathode-ray tube (CRT) and the picture tube of a television set are one and the same. ">

Although you may not be aware of this fact, the CATHODE-RAY TUBE shown in figure
2-17 is, in all probability, the one tube with which you are most familiar. Before you
started your study of electronics, you probably referred to cathode-ray tubes as picture
tubes. The cathode-ray tube (CRT) and the picture tube of a television set are one and the
same.

Figure 2-17. - Cutaway view of a typical CRT.

Cathode-ray tubes are used in more applications than just television. They can be
considered as the heart of the many types of information.

Cathode-ray tubes have one function that cannot be duplicated by any other tube or
transistor; namely, they have the ability to convert electronic signals to visual
displays, such as pictures, radar sweeps, or electronic wave forms.

All CRT's have three main elements: an electron gun, a deflection system, and a screen.
The electron gun provides an electron beam, which is a highly concentrated stream of
electrons. The deflection system positions the electron beam on the screen, and the screen
displays a small spot of light at the point where the electron beam strikes it.

THE ELECTRON GUN

The ELECTRON GUN is roughly equivalent to the cathodes of conventional tubes.
The cathode of the electron gun in the CRT is required not only to emit electrons, but
also to concentrate emitted electrons into a tight beam. In the electron tubes that you
have studied, the cathode was cylindrical and emitted electrons in all directions along
its entire length. This type of cathode is not suitable for producing a highly
concentrated electron-beam. The cathode of the CRT consists of a small diameter nickel
cap. The closed end of the cap is coated with emitting material. This is shown in figure
2-18. Because of this type of construction, electrons can only be emitted in one
direction. Notice that the emitted electrons shown in figure 2-18 are leaving the cathode
at different angles. If these electrons were allowed to strike the screen, the whole
screen would glow. Since the object of the electron gun is to concentrate the electrons
into a tight beam, a special grid must be used. This special grid is in the form of a
solid metal cap with a small hole in the center. The grid is placed over the emitting
surface of the cathode and charged negatively in relation to the cathode. The dotted lines
represent the direction of cathode emitted electron repulsion, as shown in figure 2-19.
Since all emitted electrons leave the cathode (point C), their paths can be identified. An
electron attempting to travel from point C to point B (downward) will instead follow the
path from point C to point E to point P. Consider an electron leaving from C in the
direction of point A (upward). Its path will be curved from point C to point P by
electrostatic repulsion. These curving electron paths are due to the negative potential of
the grid coupled with the high positive potential of the anode. The potential of the anode
attracts electrons out of the cathode-grid area past point P toward the screen. The grid
potential may be varied to control the number of electrons allowed to go through the
control-grid opening. Since the brightness or intensity of the display depends on the
number of electrons that strike the screen, the control grid is used to control the
brightness of the CRT.

Figure 2-18. - CRT cathode.

Figure 2-19. - Operation of the CRT grid.

The proper name, BRIGHTNESS CONTROL, is given to the potentiometer used to vary
the potential applied to the control grid. The control grid actually serves as an electron
lens. It is this electronic lens that you adjust when you turn up the brightness control
on your TV set. Notice that the effect of the grid is to focus the electron beam at point
P in figure 2-19.After passing point P, the electrons start to spread out, or diverge,
again. Therefore, it becomes necessary to provide some additional focusing to force the
electrons into a tight beam again. This is done by two additional positively-charged
electrodes as shown in figure 2-20.The first electrode is commonly called the FOCUSING
ANODE. Generally, the focusing anode is charged a few hundred volts positive with
respect to the cathode. Electrons emitted by the cathode are attracted to the focusing
anode. This is the reason that they travel through the small hole in the grid. The second
electrode, called the ACCELERATING ANODE, is charged several thousand volts
positive in relation to the cathode. Any electrons approaching the focusing anode will
feel the larger electrostatic pull of the accelerating anode and will be bent through the
opening in the focusing anode and will travel into the area labeled D. You might think
that once an electron is in this region, it is simply attracted to the accelerating anode
and that is the end of it. This does not happen. Because the accelerating anode is
cylindrical in shape, the electrostatic field radiating from it is equal in all
directions. Thus, an electron is pulled in all directions at once, forcing the electron to
travel down the center of the tube. Then, the electron is accelerated into the
accelerating anode. Once it passes the mid-point (point E), it feels the electrostatic
attraction from the front wall of the accelerating anode, which causes it to move faster
toward the front. Once the electron reaches point F, equal electrostatic attraction on
either side of the opening squeezes it through the small opening in the front of the
anode. From there, it is joined by millions of other electrons and travels in a tight beam
until it strikes the screen (point S).

Figure 2-20. - Electron-beam formation In a CRT.

THE CRT SCREEN

The inside of the large end of a CRT is coated with a fluorescent material that gives
off light when struck by electrons. This coating is necessary because the electron beam
itself is invisible. The material used to convert the electrons' energy into visible light
is a PHOSPHOR. Many different types of phosphor materials are used to provide
different colored displays and displays that have different lengths of PERSISTENCE
(duration of display).

In one way, the CRT screen is similar to a tetrode vacuum tube. Both suffer from the
effects of secondary emission. In order to reach the screen, electrons from the cathode
are accelerated to relatively high velocities. When these electrons strike the screen,
they dislodge other electrons from the material of the screen. If these secondary emission
electrons are allowed to accumulate, they will form a negatively-charged barrier between
the screen and the electron beam, causing a distorted image on the CRT screen. The method
used to control secondary emission, which you are already familiar with, i.e., a
suppressor grid, is not practical in CRT's. Instead, a special coating called an AQUADAG
COATING is applied to the inside of the tube as shown in figure 2-21. This coating is
composed of a conductive material, such as graphite, and has the same high-positive
potential applied to it that is applied to the accelerating anode. This allows the aquadag
to perform two functions. First, since the aquadag coating is positive, it attracts the
secondary emitted electrons and removes them. Second, because the aquadag is operated at a
high-positive potential and is mounted in front of the accelerating anode, it aids in the
acceleration of electrons toward the screen.

Figure 2-21. - Aquadag coating in a CRT.

Before going on, let's review what you have already learned about CRT operation.

Electrons are emitted from a specially constructed cathode and move toward the front of
the CRT. The number of electrons that leave the area of the cathode is determined by the
cap-shaped grid. In addition, the grid concentrates the emitted electrons into a beam. The
electron beam is focused and accelerated toward the screen by two electrodes: the focusing
anode and the acceleration anode. The electron beam strikes the screen and causes a bright
spot to appear at the point of impact. Any electrons released by secondary emission are
removed from the tube by the aquadag coating.

DEFLECTION

At this point, you have a bright spot in the center of the CRT screen as shown in
figure 2-22. Having watched TV, you know that a TV picture consists of more than just a
bright spot in the center of the picture tube. Obviously, something is necessary to
produce the picture. That something is called DEFLECTION. For the CRT to
work properly, the spot must be moved to various positions on the screen. In your TV set
for example, the spot is moved horizontally across the CRT face to form a series of
tightly packed lines. As each line is displayed, or traced, the electron beam is moved
vertically to trace the next line as shown in figure 2-23. This process starts at the top
of the tube and ends when the last line is traced at the bottom of the CRT screen. Because
the beam is swept very quickly across the CRT and the phosphor continues to glow for a
short time after the beam has moved on, you do not see a series of lines, but a continuous
picture.

Figure 2-22. - Impact of an electron beam on a CRT screen.

Figure 2-23. - Deflection of an electron beam across a TV screen.

These same principles also apply to the CRT used in your use of your major tool: the OSCILLOSCOPE.
Remember, the unique function of a CRT is to convert electronic (and electrical) signals
to a visual display. This function of a CRT is used in the oscilloscope to show the
waveform of an electronic signal. To help you understand better how an oscilloscope works,
we will discuss the type of deflection used in oscilloscopes. Bear in mind that the
following discussion is only about deflection; we will cover the actual operation of an
oscilloscope in a later NEETS module that deals specifically with test equipment.